1. Field of the Invention
The present invention relates to an optical beam scanning apparatus in the field of the technique of recording images by scanning optical beams such as laser beams. More particularly, the present invention relates to an optical beam scanning apparatus of the type effective for a system where a combination of large format and a small focussed beam diameter is required, such as a plotters for graphic arts or a film plotter for generating printed-circuit-board artwork.
2. Description of the Related Art
Optical beam scanning apparatus of this kind includes an image recording method which is referred to as an "Internal drum scanning configuration: and records an image by rotating and scanning optical beams inside a cylinder in its peripheral direction with respect to a photosensitive material fitted to a cylindrical surface of a fixed drum (hereinafter referred to as the "cylinder") (Japanese Unexamined Patent Publication No. 63-158580).
According to this configuration, the photosensitive material is loaded to the cylindrical surface of the cylinder. The optical beams from a light source are allowed to be incident along a central axis of the cylinder, and are reflected in a rectangular direction inside the cylinder by an optical reflection device (hereinafter referred to as the "rotary reflection device") such as a reflecting mirror or a rectangular prism which is rotated by a motor with the central axis of the cylinder being the axis of rotation. The optical beams are then condensed by a condenser lens in an optical path, and the optical beams thus condensed are scanned (hereinafter primary scanning) on the photosensitive material loaded to the cylindrical surface of the cylinder. The motor, the rotary reflection device and the condenser lens are moved in an axial direction by a driving mechanism (hereinafter subsidiary scanning), and in this way, an image is recorded (exposed).
The photosensitive material can be fitted to the cylindrical surface by retaining it on the inner surface of the cylinder, or by retaining it on an outer surface of a cylinder made of a transparent member.
According to this configuration, the optical beams which are incident substantially parallel to the axis of rotation are reflected substantially in the rectangular direction by the rotary reflection device rotating on the central axis of the cylinder as the axis of rotation thereof, so that the photosensitive material loaded to the cylindrical surface can be scanned and exposed.
The reflecting direction of the optical beams is not necessarily right angle. If they are reflected in a direction different from the right angle, the distance between the reflection point and the exposure point becomes large, and the size of the optical beams on the cylindrical surface in the axial direction of the cylinder becomes large, as well. The reflecting direction may differ from right angle within such a range where the increases in the distance and the size described above do not exert adverse influences in practice.
The advantage brought forth by this configuration is that a rotating speed can be increased and hence, a recording speed can be increased more easily because a moment of inertia in this system is smaller than that in a rotating drum configuration.
Further, the photosensitive material can be held stably in place because it is fixedly hold on the cylinder. Subsidiary scanning can be carried out by sliding the photosensitive material on the cylindrical surface but in this case, there occur the problems in that a subsidiary scanning speed becomes unstable due to friction and the photosensitive material is likely to be damaged. Unless sliding is effected smoothly, floating of the photosensitive material might occur. In the case of multiple beam recording, the photosensitive material must be kept stably so that exposure by the final beam of multiple beams can be correctly followed by the exposure by the leading beam of a subsequent primary scanning.
Subsidiary scanning may be effected by moving a part, or the whole, of an optical system including the rotary optical means in the axial direction of the cylinder. It may be effected, too, by fixing the optical system and moving the cylinder having the photosensitive material fixed thereto as a whole, on the contrary.
Another advantage of this configuration is that a relatively large recording format can be obtained by the use of a condenser lens having a small focal length in comparison with a so-called "flat-field scanning method".
The advantage obtained by the use of a lens having a small focal length is that a small condensed beam diameter necessary for recording high image quality can be obtained easily, and the scanning line interval can be reduced, too. Furthermore, it is easy to keep the beam diameter and the beam shape on the cylindrical surface substantially constant throughout substantially the full periphery by accurately adjusting the axis of rotation of the rotary reflection device with the central axis of the cylinder. Accordingly, image quality can be improved more easily.
A scanning width of from about 70 to about 80% of the full peripheral length of the cylinder is easily obtained even when limitations from mechanical arrangement of inlet and outlet openings for the photosensitive material, the rotation mechanism of the rotary reflection device and the mechanical arrangement of the subsidiary scanning mechanism, and so forth, are taken into consideration. The focal length of the condenser lens is somewhat greater than the radius of the cylinder, and a scanning width about three times the focal length can be obtained easily. Accordingly, a recording size, too becomes sufficiently large.
By the way, the condenser lens may be disposed in the optical path between the rotary reflection device and the photosensitive material, and may be rotated integrally with the rotary reflection device. In this case, there occurs a disadvantage that the moment of inertia of the rotating members increases, but the focal length of the condenser lens can be further made smaller than the radius of the cylinder and a smaller condensed beam diameter can be obtained more easily. On the contrary, there is the advantage that the diameter of the beams incident into the condenser lens can be reduced in order to obtain a predetermined condensed beam diameter.
The explanation given above can be summarized as follows. In accordance with the internal drum scanning configuration, the moment of inertia of the rotating members is small, so that the rotating speed can be increased easily and hence, the recording speed can be increased easily. It is easy to simultaneously satisfy the requirements for image quality and the recording size, and the size of the apparatus can be reduced easily to obtain the same recording size.
As described above, the optical beam scanning apparatus according to the conventional internal drum scanning configuration has the advantages described above and can easily increase the rotating speed and hence, the recording speed. Nonetheless, in order to further increase the recording speed by a single optical beam, the rotary reflection device must be rotated at an extremely high revolution speed which usually has practical limitation. To further increase the recording speed, therefore, a multiple beam scanning method is used in combination. In this case, however, the use of multiple beams is difficult, and the recording speed cannot be increased so easily.
The multiple beam scanning method is the system which utilizes a plurality of optical fibers, etc., as a light source, and effects primary scanning several times per rotation by the use of a plurality of optical beams.
The reason why the multiple beam scanning method is difficult to use is because when a plurality of optical beams are reflected by the rotary reflection device to scan on the cylindrical surface, these optical beams cross one another at two oppositions on the cylindrical surface and do not form parallel scanning lines necessary for image recording.
Even when recording is carried out only within a part of the range of the cylinder but not throughout the full periphery, a plurality of optical beams undergo curving, even though they do not cross one another, and are not suitable for multiple beam scanning for image recording.
To solve this problem Japanese Unexamined Patent Publication Nos. 59-119960 and 57-151933 disclosed a method which mounts a plurality of light sources, or a plurality of light sources together with their driving circuits, on a rotary member disposed inside a cylinder, and rotates the rotary member with the central axis of the cylinder being an axis of rotation thereof, so as to accomplish multiple beam scanning.
According to this method, however, it is necessary to mount a plurality of light sources, an electronic circuit for modulating them, a power generator for supplying power to the electronic circuit, a power source stabilization circuit, and so forth, to the rotary member. Furthermore, a mechanism for stably holding these circuits on the rotary member must be disposed. Accordingly, when the rotary member rotates, its moment of inertia is great, and there is a limit to obtain a high rotating speed.